Mac Pro 1.1 not detecting theorically compatible RAM

I have bought some compatible 4GB RAM modules, actually 2 x4 dimm packs of different brands, and I'm totally unable to get the machine to boot properly with those installed. I have tried them alone, combined with some of the RAM the machine came with with no success. For information, the machine came with 4x512MB Kingston modules and 4x512MB Hynix module.

As soon as I remove then, the machine boots flawlessly.

As I'm not very familiar with mac computers, and because first generation Mac Pros seems to be strange beast -i.e. 32 EFI for a 64 bits system-, perhaps I'm missing something.

here's a tip for you that mac the ram needs to be installed in pairs of like ram so if you put one 4 gig with a 512 it won't work right. if you have pairs of 4 gig modules install them together that might help? or not

Which diagnostic LEDs on the memory riser cards light up when you have the new memory installed?
Try just one pair, on one riser card, with ALL other RAM sticks removed, until you get this sorted out.
One pair, equal capacity, and same vendor/manufacturer.
Double-check that both sticks have the same ID label.
One pair, should be in slots 1 & 2, on the same riser card. (as showing in the good image from the post above )

so it works with the default 512mb sticks but not the new ones?
have you tried just 2 x 512 sticks then swapping them for just 2 of the new sticks in the same slots?
in the past iv had problems from dust in the slots so thats also a thing.

I already tried that with no success. All leds on the mem rise light briefly on start, then all get off.

Click to expand...

The memory LEDs will flash on once at boot. That is completely normal.
But, if you have a problem with memory, the power light (above your power button) will begin to flash, either once/second, or repeating pattern of 3 flashes. That memory error should also be echoed with the problem memory slot LED will come on.
So, with no error LEDs beyond the initial power on, yet won't boot until you have the original, working memory sticks installed, your post makes no sense - as you should show a detected memory problem (some kind of error lights)

However - you could have a possible answer, by trying a pair of 2GB sticks.
Remember that your MacPro1,1 is originally rated by Apple at a maximum of 16GB, which would be 8 x 2GB.
And, the replacement CPUs might not be compatible with 4GB sticks (could happen, I suppose)
That CPU change may be important, thanks for sharing...
That could also explain why you have 4GB sticks from 2 different vendors, and none of those apparently working in your MacPro.

I agree about this not making much sense, but you have two different brands or the 4GB sticks, and either should work.
Not sure if "cheap" and "FB-DIMMs" will fit together, but good luck in your search.

Try going with a supplier that is well-known to support the Mac quite well, like Data Memory Systems

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